just download the sequence of your gene and look, where the primers anneal and from that calculate the size of your amplicon.E.g. if your fw primer anneals to +25 nt and rev primer anneal to +450, than your amplicon will have 450-25 = ~425 nt

JackBean wrote:just download the sequence of your gene and look, where the primers anneal and from that calculate the size of your amplicon.E.g. if your fw primer anneals to +25 nt and rev primer anneal to +450, than your amplicon will have 450-25 = ~425 nt

but in most cases, primers are small, sometimes they can anneals to several site, what should i do in this case?

the primers should be long enough to be quite unique at least for your species. At PubMed's BLAST is some tool for designing primershttp://www.ncbi.nlm.nih.gov/tools/prime ... =BlastHomebut I have never used it. Anyway, if you already have your primers, just use BLAST to see, where can it anneal.Also, as you use two primers, you highly decrease the probability of inspecific amplification